1. Field of the Invention
The present invention relates to an induction heating apparatus, and more particularly, to a safety apparatus therefor.
2. Description of the Related Art
An electrophotographic image forming apparatus generally includes a fixing device for fixing a toner image transferred onto a recording material such as paper by applying heat and pressure. Although a heating system using a ceramic heater or a halogen heater has been commonly used as a configuration of the fixing device, an electromagnetic induction heating system, which quickly generates heat, has been used in recent years.
FIG. 21 illustrates a configuration of a power supply device 100 for supplying power to a fixing device 7 using an induction heating system. The power supply device 100 illustrated in FIG. 21 is supplied with alternating-current (AC) power from a commercial power supply 500 when a power supply switch 501 is turned on. The power supply device 100 includes a diode bridge 101, a filter capacitor 102, resonance capacitors 105 and 106 forming a resonance circuit, and a coil L. The power supply device 100 further includes first and second switch elements 103 and 104, a driving unit 112 for driving the two switch elements 103 and 104 by driving signals 121 and 122, a voltage detection unit 115 for detecting an input voltage, and a current detection unit 116 for detecting an input current. The voltage detection unit 115 and the current detection unit 116 respectively detect an input voltage Vin and an input current Iin that are supplied from the commercial power supply 500, and output their detection values Vs and Is to a central processing unit (CPU) 400 serving as a control unit. Relationships of Vs=αVin (α is a coefficient) and Is=βIin (β is a coefficient) hold.
The CPU 400 calculates input power from the input voltage detection value Vs and the input current detection value Is from the power supply device 100. The CPU 400 determines the driving frequencies of drive pulse signals 131 and 132, which are sent out to the driving unit 112 within the power supply device 100, based on the result of the calculation and a temperature detection value T from a temperature detection unit 114 for detecting the temperature of a conductive heating element FB. The power supplied to the fixing device 7 changes according to the driving frequencies, and the temperature of the conductive heating element FB also changes.
The driving unit 112 amplifies the drive pulse signals 131 and 132, and outputs the driving signals 121 and 122. The switch elements 103 and 104 are alternately turned on/off, respectively, according to the driving signals 121 and 122, to supply a high-frequency current to the coil L. When the high-frequency current is caused to flow through the coil L, an eddy current is induced on a surface of the conductive heating element FB by a generated AC magnetic field so that Joule heat is generated. Therefore, the conductive heating element FB generates heat.
In the induction heating system, the CPU 400 performs power control and temperature control. When the CPU 400 runs away, or an abnormality or the like occurs in the temperature detection unit 114, however, the switch elements 101 and 102 are not appropriately controlled. More power than necessary may be supplied to the fixing device 7. As a result, the temperature of the fixing device 7 excessively rises.
In order to prevent the temperature of the fixing device 7 from thus excessively rising, an input limiting unit (not illustrated) for limiting an input current is provided independently of control by a CPU, as discussed in Japanese Patent Application Laid-Open No. 2007-286495. The input limiting unit temporarily or completely stops the operation of a driving unit when the input current continuously exceeds an input current limit value for a predetermined period of time or more. The input current limit value varies according to a voltage detected by a voltage detection unit. More specifically, when the input current rises to the limit value, the input limiting unit stops outputting a drive pulse so that input power is limited.
Commercial power supplies are classified into a 100-V commercial power supply and a 200-V commercial power supply according to areas. In Japanese Patent Application Laid-Open No. 2007-286495, the input current limit value is expressed by a primary expression of an input voltage detection value. Therefore, an input power limit value is expressed by a secondary function of the input voltage detection value. FIG. 4A illustrates the circuit configuration of an input current limit value calculation circuit, and FIG. 5A is a graph of an input current limit value.
In FIG. 5A, an input current limit value I2 is expressed by the following equation:I2=−(R2/R1)×Vs+(1+R2/R1)×V2
Accordingly, an input current limit value Ilim is expressed by the following equation:Ilim={−R2/R1)×Vs+{(1+R2/R1)×V2}}/βHere, β is a transform coefficient for detecting the input current Iin as the input current detection value Is by the current detection unit 116, and a relationship of Iin=Is×β holds.
α is a transform coefficient for detecting the input voltage Vin as the input voltage detection value Vs by the voltage detection unit 115, and a relationship of Vin=Vs×α holds.
Therefore, an input power limit value Plim is expressed by the following equation:Plim=Vin×{−(R2/R1)×αVin+(1+R2/R1)×V2}/β
As an example, R1=33 kΩ, R2=68 kΩ, V2=1.7, α=0.011, and β=0.22. An input voltage range may be relatively narrow. For example, the input voltage range is 85 V or more and 120 V or less. In such a case, an input power limit value is 1260 W when the input voltage Vin is 85 V, and is 1350 W when the input voltage Vin is 120 V. Therefore, a difference between the upper limit and the lower limit of the input power limit value is 90 W.
Then, the input voltage range may be 85 V or more and 200 V or less. In such a case, the input power limit value is 600 W when the input voltage Vin is 200 V. Therefore, a difference in the input power limit value from when the input voltage Vin is 120 V is 750 W (FIG. 22).
Even if 1200 W is required as the power of the fixing device 7 when the input voltage is 200 V, therefore, the input power is limited at a time point where it reaches 600 W. Therefore, no required power is obtained.
In order to limit the power at an appropriate power value, there is a method for providing a multiplier within the input current limit value calculation circuit and putting the detected input voltage and input current into the multiplier. However, the cost of the multiplier is high, and a circuit configuration also becomes complicated.